In all weaving, an initially flat array of longitudinally extending warp threads is divided into at least two interspersed groups which are separated in opposite directions from the starting plane to define between the separated warp groups an elongated diamond shaped space, known as a "shed", through which the weft or filling is inserted, the direction of separation of the warp groups being reversed in a given order after each such weft by means of a harness motion with the result that the warp threads are entwined in sinuous fashion around successive filling threads to form the woven fabric. Traditionally, the weft is carried in coiled form upon a bobbin held within a shuttle, and as the weaving progresses, the shuttle is propelled alternatively back and forth through the shed on the upper surface of a beam-like lay which carries a comb-like reed projecting upwardly therefrom and rocks back and forth to press or "beat up" each new weft by means of the reed against the working end or "fell" of the fabric being woven. In the traditional loom, bobbin propulsion was accomplished by means of so-called picker sticks mounted on the loom adjacent opposite side edges of the warp for pivotal movement about their lower ends and driven to alternately impact their upper ends against the shuttle. Obviously, this conventional design was subject to inherent limitations as to achievable shuttle speed and was, moreover, accompanied by substantial disadvantages; namely, deafening operating noise as well as risk of breakage of picker sticks or other damage to equipment and of danger to operating personnel when, as occasionally happened, the shuttle escaped its containment and became an uncontrolled projectile. In order to overcome these inherent problems in bobbin type weaving, the prior art has explored various alternatives, and in the past decade or so, increasing attention has been directed to the possibility of impelling the weft thread through the shed by means of a jet of fluid. Jets of water have been found to be a relatively manageable projection medium, but water is a possible cause of corrosion and limits the choice of yarn material; thus there are significant advantages in the use of a gaseous fluid. While gases other than air can in theory serve equally well, cost considerations dictate the choice of air as the only practical gaseous propelling medium; consequently, this mode of weaving will hereinafter be referred to for convenience as "air weft insertion", although the use instead of other gases is, in principle, intended to be included.
In general, air projection techniques that have been used in past air weft insertion systems fall into two basic categories. In one type, the weft end is initially projected by means of a pressurized air from a nozzle situated outside and adjacent one side of the warp shed which serves to initially accelerate the weft end and starts its travel through the shed. The propulsion forces of existing nozzles is severely limited in terms of the attainable length of projection of the weft end and hence, in this type, a plurality of "booster" or supplemental jet nozzles is provided at spaced intervals through the shed, such nozzles being inserted within and removed in various ways from the shed interior via the clearance between the warp yarns. The aggregate of the propulsion forces of this multi-stage sequence of nozzles can be sufficient to convey the weft thread across the full width of the loom.
While this approach has proved generally feasible in practice, it too is faced with definite disadvantages, viz, the requirement for carefully controlled timing of the sequence of nozzle action plus excessive consumption of compressed air and thus poor economic efficiency.
In order to avoid the need for booster nozzles disposed at intervals through the shed, an alternative approach has been developed in a second type which utilizes a single exterior insertion nozzle in conjunction with a weft guidance "tube" situated within the shed. Since during weaving, the groups of warp threads must shift up and down past one another, the presence of any continuous body within the shed during shedding is out of the question. Therefore, an "interrupted" weft guidance tube is used, taking the form of a plurality of generally annular segments, each shaped to sufficiently narrow thickness in its axial dimension as to pass between adjacent warp threads arranged in an axially aligned position so as to constitute together a lengthwise interrupted tubular member extending substantially the entirety of the shed width. Each annular segment has a slot-like exit opening at a point on its periphery to allow integral egress of the inserted weft thread when the guidance tube is withdrawn below the shed. When the weft thread is projected by the exterior nozzle into one end of this interrupted guidance tube, the projection force imparted to the thread by the nozzle appears to be substantially enhanced so that the distance the weft thread is propelled by this force can be significantly increased compared to the nozzle alone.
The reasons why the interrupted guidance tube extends the projection force of the nozzle are not totally understood at present. The adjacent segments of this tube are separated by clearance spaces which are sufficient to permit pressurized air delivered into one end of the tube to disperse to the outside atmosphere while the interior edges of the bore of the segments should present considerable frictional resistance to movement of an air jet therethrough; from this standpoint the effect of such a tube might be expected to be negative. On the other hand, ambient air could be entrained from the ambient atmosphere into the interior of the tube through the same intersegment spaces with the possible effect of augmenting the propelling forces. In any event, it is established that the addition of a weft guidance tube generally as described above substantially increases the distance a weft thread can be projected with a jet of compressed air emitted from a nozzle.
Numerous improvements have been suggested in this second type of air weft insertion system which in general have focused upon refinements in various aspects of the system, including enhancing the effect of the guidance tube by means, for instance, of arrangements capable of temporarily reducing the clearance space between the segments thereof during the weft insertion phase of the cycle or by developing superior aerodynamic characteristics for such elements, optimizing the delivery of a measured weft length to the insertion nozzle through a variety of weft measuring and storage devices intended to minimize the resistance of the weft length to propulsion and hence utilize to maximum advantage the thrust capability of a given nozzle, and the like. With rare exceptions, the prior art efforts in this type of system have given little attention to the fundamental behavior of the air delivery stream itself.
It is known according to aerodynamic theory that the thrusting force (dF applied by a moving gaseous stream to an element disposed therein with a given unit length (dx) and a circumference (.pi.D) is determined by the equation: EQU dF=C.sub.f .multidot.1/2.rho.(V.sub.g -V.sub.e).sup.2 .multidot..pi.D.multidot.dx I
where .pi. is the density of the gaseous medium, C.sub.f is a factor varying with the condition of the element and is roughly constant for a given thread, V.sub.g is the velocity of the medium, V.sub.e is the velocity of the element and D is the diameter of the element. In a given system the diameter and factor C.sub.f will ordinarily be fixed; hence, thrusting force is essentially a function of the density of the medium and the square of the difference in velocity between the moving gaseous medium and the element. In inserting weft in the shed of a loom, the weft will normally be stationary prior to the insertion so that V.sub.e becomes zero and the starting thrusting force, therefore, is essentially proportional to .rho.V.sub.g.sup.2.
The practical application of this result is somewhat complicated by the generally opposing behavior of velocity and density in the system in question. At velocities below sonic speed (sonic speed being referred to as a Mach No. of 1 or "Mach 1"), velocity varies with the square root of the head pressure so that in order, for example, to double the velocity the pressure must be quadrupled. At a given head pressure, as the air accelerates along the nozzle, the pressure drops and is accompanied by a decrease in density according to the relationship required for adiabatic processes. When V.sub.g reaches sonic velocity in the throat of the nozzle, the rate of change in .rho. has become exactly equal to the rate of change of V.sub.g and .rho.V.sub.g thus has its maximum value at the throat for a given supply pressure. At all velocities above sonic velocity, .rho. decreases more rapidly than V.sub.g increases. From Mach 1 to Mach 1.414, the relative rates of change are such that .rho.V.sub.g.sup.2 continues to increase, while above Mach 1.414 .rho. decreases at sufficiently higher rates than V.sub.g increases that .rho.V.sub.g.sup.2 becomes smaller so that for example .rho.V.sub.g.sup.2 is approximately the same at Mach 2 as at Mach 1.
As the head pressure is made greater, V.sub.g increases, as mentioned, until sonic velocity is achieved, but further increases in head pressure produce increases only in the ultimate level of .rho. in the throat and not in V.sub.g. That is, the highest throat velocity possible is Mach 1 irrespective of increases in pressure, which only serve to make the gas more dense. Acceleration of the gas to supersonic speed is possible only by increasing the volume of the space downstream of the throat to allow the densified gas to expand and decrease .rho., and hence make it possible for V.sub.g to increase. If the nozzle throat opens directly into the ambient atmosphere, the gas can expand randomly for a short distance while if the nozzle has a convergently contoured section below the throat (and thus forms a so-called super-sonic nozzle) the gas can expand in a controlled fashion.
For gas velocities above Mach 1 downstream of the throat, the pressure increase required for a given change in Mach No. is a geometrical rather than a linear function. For example, the theoretical ratio of head pressure to ambient pressure for Mach 1 is approximately 1.9, for Mach 1.414 approximately 3.25/1, for Mach 2 about 7.9/1 and in practice should be somewhat higher.
Clearly from these technical considerations, increasing V.sub.g by increasing head pressure definitely appears to be an unpromising way in terms of cost effectiveness of increasing the thrusting force dF in the above-equation since at below sonic speeds a given theoretical increase in V.sub.g requires the head pressure to be increased by the square of the difference and this disproportionality between velocity change and head pressure change comes even worse at above sonic velocities. Further, the gas velocity in the throat can in any case never exceed sonic speed and the essential thrusting force .rho.V.sub.g.sup.2 itself is subject to limiting value at the low level of Mach 1.414 and can thereafter only decline.
To the apparent technical cost disadvantage of high nozzle pressures must be added the practical necessity for pressurized air used for weft insertion to be free of contaminants such as oil and dust particles. The production of such clean air requires special centrifugal compressors and/or special filtration devices which substantially increase the machinery investment for a given installation.
For these and other reasons, prior art workers in weft insertion systems have without known exception accepted the principle of a low pressure air supply and low air jet velocity as unavoidable conditions and have striven to use these given conditions with maximum effectiveness, placing their concentration on other techniques, as stated.
In any weaving operation, each weaving cycle divides into two main phases, the weft insertion phase, which occurs generally at the rearward end of the lay rocking motion, and the beat up phase, which occurs when the lay is rocked forwardly to the other limit of its arcuate path to pack, or beat up, the newly inserted weft end (or pick) against the fell of the already woven fabric, with the fabric being stepwise advanced as needed to maintain the fell at a fixed location. Various attempts have been made to shorten the beat up phase, so as to thereby increase overall weaving speed, by employing, for example, special mechanical drives designed to accelerate lay movement during beat up, and specially constructed lays with shortened pivot supports and reduced mass to shorten the arc of lay travel and lessen inertial forces involved in driving the lay, all of which can be advantageous. There are, however, inherent limitations on how far beat up time can be reduced in this way; consequently, the achievement of truly high speed loom operation, i.e. in the order of 1,000 weft insertions or picks per minute, is ultimately possible only by taking less time to insert the weft itself. Specifically, at 1,000 picks per minute, only a total of 60 milliseconds is available for an entire weft insertion or picking cycle, i.e. the lapsed time from one beat up to the next. Prior art air weft insertion systems normally require at least 50-60 milliseconds for weft insertion alone, apart from the beat up phase, and have, therefore, been inherently limited in operating speeds.
There are presently in use or available for use in the textile industry several millions of existing shuttle-type looms which were designed for operation at speeds of up to about 150-200 picks per minute and cannot be adapted for high speed operation without a virtual complete rebuilding. However, with a fairly modest amount of mechanical modification, such looms can be driven at speeds of about 400 cycles per minute. At this speed, the period required for one complete cycle is 150 ms, and in theory, weft insertion times in the order of 50-60 ms as characteristic of the prior art might be tolerable in a cycle of this duration. However, at insertion times of this order, loom operation would become somewhat critical due to the large proportion of total cycle time consumed by the insertion time, and might require special "dwell motions" for this purpose. Consequently, it would be of a definite benefit in the conversion of existing shuttle looms to air weft insertion for the weft insertion time to be reduced substantially below the prior art level and thereby impart greater flexibility to and reduce criticality in the operation of such converted looms.